This quote from AGEWiki describes frontage impact from commanding general.

In open terrain only (clear/prairie/desert/wood), the Units Quotas are modified by leader (rank)*(offensive/defensive rating) depending whether in offensive or defensive posture:

Combat Units Quota: (+25 points)*[(rank)]+(off/def rating)]

Support Units Quota: (+10 points)*[(rank)]+(off/def rating)]

However, I didn't found where to mod this in game files. Is it hardcoded in game exe file? I found this formula to work well in Cw2 where clear terrain has 180 base frontage points, but in EAW where terrains generally have less frontage points (48 points clear terrain) I don't know if it is still vallid. If it is, a good general could impact frontage over 5 times which I don't think is likely.

I'm speaking for RoP here. I've found no way to change the bonus amount of frontage that C-in-C generals provide in open terrain. However, I've simply turned off the whole "bonus-frontage"-feature in my mod by declaring all terrain as "non-open" in the terrain files.

If you want to keep the bonus but fine-tune it, I think you could also work your way around by calibrating the frontage that each territory provides (in each weather) and that each unit needs based on the (hard-coded?) generals' frontage boni.